1. Field of the Invention
The present invention relates to a cell comprising a spirally wound electrode body, for example, an alkaline secondary cell such as a nickel-hydrogen occlusion alloy cell and a nickel-cadmium cell. In particular, the present invention relates to an improvement of the spiral structure of the electrode body to increase the capacity, reliability and productivity of the cells, and to lower the production costs.
2. Prior Art
The spiral structure of electrodes which is employed in the nickel-hydrogen occlusion alloy cells or nickel-cadmium cells is shown in FIG. 11, in which one positive electrode 1 and one negative electrode 2 are spirally wound with interposing a separator 3 between them. That is, the positive and negative electrodes 1 and 2 are formed in certain thicknesses and wound to assemble an electrode body 4 having the spiral structure as shown in FIG. 11.
It is necessary for the alkaline secondary cells such as nickel-hydrogen occlusion alloy cells or nickel-cadmium cells to maintain a ratio of the electric capacity of negative electrode to that of positive electrode of at least 1.0, preferably at least 1.2. This ratio is not an overall ratio in the cell but should be maintained at facing areas between the spirally wound positive and negative electrodes. Therefore, the cells comprising the conventional spirally wound electrode body are designed based on the ratio of the electric capacity of negative electrode to that of positive electrode in the part where the both surfaces of the negative electrode face the positive electrodes, that is, in the second turn of the negative electrode. For this reason, the innermost and outermost turns of the negative electrode have the electric capacities larger than the necessary values.
The innermost and outermost turns of the negative electrode of the conventional spirally wound electrode body carry the layers of electrode active materials on both sides of the substrate although only one side faces the positive electrode in those turns. Thus, the electrode active material on the other side is not used effectively, and as the result, the inner volume of the cell is not used fully.
Small cells have a structure such that the outermost turn of the spirally wound electrode body is the negative electrode which is in contact with the inner wall of a cell case to form an electrical connection. Thus, the raised parts of the layer of the electrode active material tend to flaw the inner wall of the cell case, and the flaws may lead to serious defects of the alkaline cells such as leakage of an electrolytic solution.
The conventional negative electrode comprises a nickel sintered substrate which is produced by coating a nickel powder paste on a perforated iron plate plated with nickel and sintering it, or a porous substrate such as a foam or fibrous metal plate which is produced by plating nickel on a urethane foam or non-woven fabric and sintering it to proceed the electrode reactions on the both surfaces. Therefore, the costs for equipments for producing the electrode bodies or the substrates increase, and an enormous amount of work is necessary for the stable production of the electrode bodies or the substrates having constant properties.